This invention relates to a method and apparatus for manufacturing spiral pipe wherein a strip of pipe forming material is bent into a helical form and the abutting edges of the bent strip are welded together.
Methods of manufacturing spiral pipe fall into three general categories: (1) methods using external holding, (2) methods using internal holding, and (3) methods that use no holding.
In the external holding method, the strip is helically bent into a round form using three sets of forming rolls arranged triangularly. While the springback of the spirally formed strip is prevented by means of a multitude of stationary external holding rolls exerting pressure from outside the pipe, the abutting edges of the strip are welded together. Thus, welding is accomplished while the unwelded bent strip is in contact with the holding rolls. Consequently, the pipe is not welded while it is completely free of load. This is, the seam is formed without relieving the elastic strain within the material of the pipe. Accordingly, when a longitudinal slit is cut in the pipe, the pipe springs back in a direction tending to open the pipe because of residual moment (hereinafter called the ring-opening; the residual moment being defined as positive). The internal holding method bends strip using similar forming rolls, and then welds together the abutting edges while exerting a force which tends to slightly expand the bent strip by means of a multitude of stationary internal holding rolls. In this case, a longitudinal cut made in the pipe causes the pipe to spring back so that the edges of the slit overlap because of residual moment (hereinafter called the ring-opening or the ring-closing; the residual moment being defined as negative). The method using no holding roll bends the strip so that the bent strip possesses the desired outside diameter after allowing for full springback, both inward and outward. With residual moment thus eliminated, the welded pipe does not spring back even if a longitudinal slit is cut therein.
As will be understood, a given amount of springback in a given direction develops in spiral pipe manufactured by most of the conventional methods, despite the intentions of the manufacturer. Therefore, no one has heretofore thought to provide residual moment in the pipe intentionally. As the use of spiral pipe becomes more widespread, however, the inventors have noticed that ignoring this residual moment is responsible for several defects, or impairment of several advantages, of the spiral pipe. For example, a positive residual moment in spiral pipe used to conduct so-called sour gas accelerates the development of stress corrosion cracks. It is therefore desirable to provide offsetting negative residual moment. The spiral pipe manufactured by the external holding method has much residual stress that develops a positive residual moment. When this type of pipe is used for a pipe line, the pressure of the liquid carried therein expands the pipe so as to lower the strength thereof. If a sheet pile is attached to spiral pipe which has negative residual moment or which has no residual moment at all, the cross-sectional shape of the pipe tends to become warped. If a suitable moment of positive residual moment exists, the same pipe will maintain its original round cross-section. It is thus necessary, depending on the use; to control the residual moment (internal elastic strain) that causes springback so that it is within a suitable range. Despite this necessity, the conventional manufacturing methods and apparatus only serve the purpose of making spiral pipe having desired diameters, and do not include means to control residual moment.
Being stationary, the conventional forming apparatus cannot freely control the amount or extent of bending. Therefore, it is difficult not only to provide positive and negative residual moment as desired but also to control the amount of either type of residual moment by means of a single manufacturing method and apparatus. Therefore, to make spiral pipe with positive and negative residual moment as desired requires at least two different types of manufacturing equipment, i.e. which respectively operate on the external and internal holding principles. Providing two different lines in a limited plant space, however, lowers equipment utilization rate, entailing an increase in capital investment and production cost and a considerable economic disadvantage.